The technique consisting in placing luminal endoprostheses and in particular stents in blood vessels affected by thromboses or on which aneurysms are developing is currently well known and widely applied, in particular in the cardio-vascular field.
However there is still a significant problem during placement and during subsequent checks on the state of patients: because of their slenderness, these endoprostheses are extremely difficult to pinpoint by X-ray endoscopy. The only part which appears under imaging is their armature, whose mass and thickness decrease with the diameter of the vessels treated.
The most striking case is that of interventions in the volume of the cranial box: the bones of the cranium constitute by themselves a significant obstacle for the absorption of radiation. Stents for the treatment of cerebral aneurysms are characterized moreover by their small diameter, in general of the order of 3 to 5 mm. The wires forming these stents are themselves extremely slender (of the order of 40 to 50 μm), so that it is possible to introduce the stent via a catheter whose outside diameter does not exceed 3 units French. When depositing a stent of this order of dimensions under X-ray monitoring, the stent, on account of the small thickness of the wires used and of the loss of contrast due to the thickness of the cranial box, is practically impossible to pinpoint, which means that the operator, for these very tricky operations, must work practically blind.
In practice, it is considered that most wires lose their radio-opacity for a thickness of the order of 0.1 mm. The use of noble metals (wires made of platinum, gold or tantalum) would make it possible to go down as regards visibility to a diameter of the order of 25 μm. Unfortunately, the mechanical properties of these metals are mediocre, so that it is not possible to use them alone: it is compulsory to associate them with other more resistant metals.
To alleviate this problem, various attempts have been formulated. U.S. Pat. No. 6,569,194 for example proposes the use of an alloy incorporating a metal such as tungsten, thereby obviously altering the mechanical performance of the armature.
US 2003/0121148 and WO 2005/028014 envisage, for stents formed on the basis of tubes sectioned by laser, the fixing, by welding, of inlays made of highly absorbent materials, such as platinum, gold or tantalum which therefore function as markers. This procedure makes it necessary to resort, during manufacture, to a very elaborate and expensive rig. The fixing of these inlays may possibly have to be done in two steps, since it is necessary to resort to an intermediate so-called priming layer to obtain sufficient adhesion. Furthermore, the excess material must be eliminated by electropolishing. Another problem which is related to this technique is the occurrence of galvanic corrosion phenomena between the welded material and the metal of the armature in the blood medium, acting as an electrolyte. The same problem appears in EP-0894481, where bits of radio-opaque metal wires are placed as markers on the armature.